1. Field of the Invention
The present invention relates to the field of telecommunications, and more particularly to an improved method and system for handing off communications between cells or sectors of a wireless communication system.
2. Description of the Related Art
Wireless cellular communications is becoming a convention method for people to communicate with one another. A conventional wireless cellular communication system in accordance with an industry standard commonly known as IS-95B, which is issued by the Telecommunications Industry Association and Electronics Industry Association (TIA/EIA) defines the way in which one type of wireless cellular communications are performed. In accordance with IS-95B, a mobile station (such as a wireless cellular mobile telephone) communicates with other mobile stations, a conventional telephone, or other such communication devices over a communications link that includes at least one cellular base station. The mobile station transmits a radio signal to the cellular base station. The cellular base station establishes a connection to a wired network that might include conventional telephone circuits (commonly known as the public switched telephone network (PSTN)).
A mobile station need only establish communication through one base station at a time in order to communicate with a device at the other end of the communications link (i.e., make a “call”). However, as a mobile station moves, the mobile station and the base station may lose the ability to communicate over the radio link. For example, if the mobile station moves outside the range of the base station or if an obstruction comes between the mobile station and the base station, the communications between the mobile and base stations will be interrupted. Therefore, the placement of base stations is planned such that there is an overlap between the coverage areas of each base station. This overlap ensures that a mobile station can contact at least one base station in every geographic point intended to be covered by the system. This is important because if the mobile loses contact with all base stations for any substantial amount of time, the call is “dropped”. Once a call is dropped, the call must be reestablished by the mobile station redialing the call.
Due to the substantial overlap between base station coverage areas, a procedure known as “soft handoff” can be performed. Soft handoff is a process in which the mobile station receives identical signals from both a first and a second base station. A mobile station will preferably enter soft handoff (i.e., signals from a second base station will be received by the mobile station) whenever a second station becomes available. Soft handoff ensures that a call is not dropped as the mobile station moves out of the coverage area of a first base station and into the coverage area of a second base station.
One conventional method for performing a soft handoff is illustrated in FIG. 1. FIG. 1 shows a mobile station 101, a first base station 103, a second base station 105, and a mobile switching center (MSC) 107. In addition, the time sequence of communications between each is illustrated as follows. Arrowheads that terminate on the vertical line 109 that descends from the mobile station 101, for example, represent signals received by the mobile station 101. Arrows that terminate without an arrow head (i.e., at originate) at the vertical line 109 represent signals that have been transmitted from the mobile station 101. Arrows that are closer to the top of the figure represent signals that are transmitted before signals represented by arrows closer to the bottom of the figure. In some instances, an arrow that is above another arrow may represent a signal that is transmitted continuously and thus may be transmitted concurrent with the signal represented by the lower arrow. For example, the traffic signal represented by arrow 111 may continue to be transmitted concurrent with the pilot strength measurement message (PSMM) that is represented by arrow 113.
As shown in FIG. 1, the traffic signal 111 is initially transmitted between the mobile station 101 and the base station 103. The traffic that is transmitted from the mobile station 101 to the base station 103 is then sent on to the MSC 107 by the base station 103. Likewise, traffic that originates at the MSC 107 is sent to the base station 103. This traffic is then transmitted from the base station 103 to the mobile station 101. When the mobile station 101 detects a pilot from the second base station 105 with sufficient power, the mobile station 101 transmits a PSMM to the first base station 103 indicating the pilot strength of all the pilots that are currently being received at a signal level that is above a predetermined threshold. In the case shown in FIG. 1, the PSMM indicates that the mobile station 101 is receiving pilot signals that are above the predetermined threshold from both the first base station 103 and the second base station 105. This PSMM is then transmitted from the first base station 103 to the MSC 107, as represented by arrow 115. The MSC 107 responds to the receipt of this PSMM by requesting the second base station 105 to allocate resources to establishing a communication link between the second base station 105 and the mobile station 101, as represented by the block 116. In addition, the MSC 107 generates a handoff direction message (HDM). The HDM is transmitted from MSC 107 to the first base station 103, represented by the arrow 117, after a time delay, represented by the arrow 119. The HDM message is then transmitted from the first base station 103 to the mobile station 101, represented by the arrow 121. The HDM indicates to the mobile station 101 that a request has been made for the second base station 105 to allocate resources to establishing a communications path between the second base station 105 and the mobile station 101.
The mobile station 101 responds to the HDM by adding the second base station 105 to the “Active Set” in the mobile station 101 and transmitting a handoff completion message (HDM) to both the first base station 103 and the second base station 105, represented by the arrows 123, 125. Both the first and second base stations 103, 105 transmit the HCM to the MSC 107, represented by the arrows 127, 129. The active set in the mobile station 101 indicates which base stations are actively in communication with the mobile station 101. Traffic will then be transmitted from the MSC 107 to the mobile station 101 through both the first and second base stations 103, 105.
This procedure works well in most cases. However, in some cases, the pilot transmitted from the second base station 105 is received by the mobile station shortly before the signals received from the first base station 103 can no longer be received by the mobile station 101. If the time delay between receipt of the PSMM 115 and the transmission of the HDM from the first base station 103 is such that the communication link between the mobile station 101 and the first base station 103 deteriorates before the HDM can be received from the first base station 103 by the mobile station 101, then the call will drop.